Solution for the selective removal of metal from aluminum substrates

ABSTRACT

The present disclosure relates to a solution for selectively removing metal, such as Ta or TaN, from a substrate, such as an aluminum containing substrate. The solution comprises an acid, such as HF or buffered HF, an ingredient comprising a fluorine ion, such as ammonium fluoride (NH 4 F), ethylene glycol, and water. A method of selectively removing metal from a substrate using this solution is also disclosed.

This application claims priority from U.S. Provisional PatentApplication Ser. No. 60/638,308 filed Dec. 22, 2004.

The present disclosure relates to a solution for selectively removingmetal, such as Ta or TaN, from a substrate, such as an aluminumcontaining substrate. Also disclosed is a method of selectively removinga metal coating without substantially damaging the exposed metal of theunderlying substrate.

The fabrication of semiconductor devices typically comprises formingmultiple layers, including a uniform photoresist coating over conductivemetallic layers. Non-limiting examples of conductive metallic layersformed as diffusion barrier layers on substrates include those ofaluminium, titanium, tungsten, tantalum, and copper, as well as oxides,borides, nitrides, carbides, and alloys thereof. These conductivemetallic films are generally formed as diffusion barriers to preventmetal in a metal layer from diffusing into an underlying dielectriclayer located on the substrate. For example, a barrier layer istypically formed between a silicon containing dielectric layer (such asSiO₂ or Si₃N₄) and an Al or Cu metal layer.

Recently, tantalum (Ta) and tantalum nitride (TaN) have been found to beexcellent alternatives to a traditional Ti-based diffusion barrier foruse in semiconductor processing, at least in part because thesematerials are effective diffusion barriers up to temperatures as high as200° C. The increased use of Ta in device manufacturing, has led to aneed for a process or a stripping solution to remove the Ta or TaN layerfrom the substrate.

For example, after performing well-known steps to obtain a microcircuit,including selective photo-etching, the removal of metals from certainareas of the microcircuit, such as a Ta or TaN layer, has typically beenaccomplished by organic stripping solutions.

Chemical removal of tantalum metal from an aluminum substrate is anextremely difficult process to execute. This is at least becausetantalum is highly resistant to most chemical attack and requires anagent capable of donating a fluorine ion to facilitate dissolution ofthe Ta containing layer. Recently, methods of wet processing electroniccomponents having tantalum containing layers thereon have been attemptedwith an oxidizing agent. For example, U.S. Published Patent ApplicationNo. 2002/0119245A1 describes a tantalum oxidizing solution, comprisingan oxidizing agent, such as hydrogen peroxide or ozone, and a fluorineion producing agent, such as HF or buffered HF, for wet processingelectronic components.

It would be apparent to one skilled in the art that fluorine ionproducing agents, such as hydrofluoric acid (HF) solutions readilyattack the exposed metal, such as aluminium. Accordingly, the substrateis typically severely eroded during the process and can be damagedbeyond repair.

For these reasons, alternative methods have been employed for thisprocess such as mechanical abrasion, sometimes referred to as “ChemicalMechanical Polishing” (CMP), which removes the tantalum but also resultsin substrate damage, particularly when anything less than the entiresubstrate is covered with Ta or TaN.

Other methods of removing Ta and TaN include sputter etching. UnlikeCMP, which is most useful when the entire substrate is covered with Taor TaN, sputter etching is generally useful for a patterned etch becausevery specific areas can be removed in an anisotropic manner. Inherentdrawbacks associated with sputtering, including the likelihood thatmaterial removed by sputtering can redeposit on exposed surfaces, havelimited the use of this technique.

SUMMARY

In view of the foregoing, there is a need to remove metal from asubstrate so as to possibly overcome at least part of one or moredrawbacks associated with the related art. For example, some aspects ofthe present disclosure may relate to the selective chemical removal ofmetal from a substrate with minimal or no damage to the substrate.

One exemplary aspect may relate to a solution for selectively removingmetal from an aluminium containing substrate. The solution may comprise:at least one acid, such as HF or buffered HF, at least one ingredientcomprising a fluorine ion, such as NH₄F, ethylene glycol, and water. Thecombination of these ingredients is found in the final solution in anamount sufficient to remove metal, such as Ta, from an aluminiumcontaining substrate.

Methods of removing metal from substrates are also disclosed. Oneexemplary method may be a method for selectively removing metal from asubstrate comprising aluminum and being at least partially covered witha metal. For example, the method may comprise contacting the substratewith a solution comprising, in % weight of the solution, 1-20% HF,buffered HF or mixtures thereof, 1-40% NH₄F, 40-95% ethylene glycol, andthe balance of water. In one embodiment, the solution further comprisesa surfactant to aid the removal of metal from the substrate.

In the disclosed method, the solution may be contacted with thesubstrate for a time sufficient to remove the metal from the substratewithout substantially reacting with the underlying aluminum substrate.The example may also include forming an aluminum ethoxide layer on atleast one portion of exposed aluminum substrate.

Aside from the subject matter discussed above, the present disclosureincludes a number of other exemplary features such as those explainedhereinafter. It is to be understood that both the foregoing descriptionand the following description are exemplary only.

BRIEF DESCRIPTION OF THE DRAWING

The accompanying drawing is incorporated in and constitutes a part ofthis specification.

The FIGURE is a schematic diagram of an exemplary embodiment of analuminum substrate including one portion with a protective layer andanother portion with a metallic coating to be selectively removed.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

As used herein, “selectively remove” (or derivations thereof) isintended to mean removing or stripping metal from a substrate withoutsubstantially reacting with or eroding the underlying substrate.Although in no way intending to be bound by the theory, it is believedthat the highly selective removal properties associated with thedisclosed solution is a function of the presence of ethylene glycol inan amount sufficient to form an ethoxide layer on at least one portionof the substrate not covered with the metal, i.e., the exposedsubstrate.

In certain embodiments, ethylene glycol may be used in combination withor completely substituted with other components. Non-limiting examplesof such components include propylene glycol, butanediol, glycerol, orany straight, or branched chain-diol or -triol from C₂ to C₆ chain.

The FIGURE shows an example of the basic mechanism for the selectiveremoval process described herein. In this embodiment, an organometalliclayer forms on an exposed surface of an aluminium-containing substrateand protects the substrate from HF attack during a stripping process. Asshown in the FIGURE, by reacting with an exposed portion of thesubstrate, ethylene glycol forms a protective layer (e.g., aluminiumethoxide) on the exposed portion of the substrate. This protective layersubstantially prevents the substrate from being attacked by a subsequentacid treatment. For example, when the substrate comprises aluminum, theethylene glycol is present in an amount sufficient to form an aluminumethoxide layer on at least one portion of the substrate not covered withmetal. In one embodiment, the amount of ethylene glycol sufficient toform an ethoxide layer may range from 50 to 60%, by weight of thesolution.

Because it does not contain a protective ethoxide layer, the metalliccoating may be susceptible to attack from an acid, such as HF. In someexamples, the process described herein may lead to the selective removalof one or more metallic coatings (e.g., undesired metallic coatings),such as copper, tungsten, titanium, tantalum, as well as oxides,borides, nitrides, carbides, alloys and mixtures thereof, withoutsubstantially contacting acid with the substrate and/or etching theunderlying substrate.

As used herein, “without substantial contacting . . . and/or etching” isintended to mean no detectable attack on the underlying substrate, asdetermined by visual inspection or weight loss measurements of thesubstrate.

In one embodiment, the acid component of the solution comprises afluorine ion, such as HF or buffered HF. In another embodiment, the acidmay comprise any well-known acid, such as an acid chosen fromhydrochloric, sulfuric, nitric, and phosphoric acids, and mixturesthereof. Any one or any combination of these acids may be present inamount ranging from 1-20%, such as from 10-15%, by weight of thesolution.

In another embodiment, the solution comprises at least one ingredientcomprising a fluorine ion. Non-limiting examples of such ingredientsinclude ammonium fluoride (NH₄F), sodium fluoride, potassium fluoride,ammonium bifluoride, sodium bifluoride, ammonium fluroborate, and bariumfluoride. Any one or any combination of these ingredients may be presentin amount ranging from 1-40%, such as from 5-20%, and even 5-15%, byweight of the solution.

The solution described herein may optionally comprise additivesgenerally used in stripping solutions. For example, the solution maycomprise one or more additives chosen from surfactants, anti-corrosionagents, pH modifiers, and the like. In one embodiment, these optionalingredients may be present in an amount up to 5% by weight of thesolution. Non-limiting examples of surfactants that can be optionallyincorporated into the disclosed solution include both straight chain andbranched primary, secondary and tertiary amines of chain length C₂ toC₁₂, amides, fluorocarbons and alkylethoxylates, including anionic,nonionic, cationic and amphoteric surfactants, and mixtures thereof.Examples of such surfactants can be found in Published U.S. PatentApplication Nos. US 2002/0119245 A1, published Aug. 29, 2002, and US2003/0114014A1, published Jun. 19, 2003, which are herein incorporatedby reference. In one embodiment, the surfactant is octylamine (C₈).Other surfactants that may be used include fluorocarbon surfactants,such as the FC range from 3M®, amides and alkyl ethoxylates.

Examples of the anti-corrosion agent include benzotriazole andpyrocatechol.

Non-limiting examples of pH modifiers that may be used to maintain a pHof the solution of less than 7, such as less than 6, even less than 5,and less than 4, include any suitable acid, such as an acid chosen fromhydrochloric, sulfuric, acetic, and phosphoric acids, and mixturesthereof.

In one embodiment, there is disclosed a solution for selectivelyremoving Ta or TaN from an aluminium-containing substrate, wherein thesolution comprises, in weight % of the solution: 1-20% HF, 1-40% NH₄F,40-95% ethylene glycol, and optionally a surfactant, with the balancecomprising water.

Also disclosed herein is a method for selectively removing metal from asubstrate using any of the previously described solutions. For example,the method comprises contacting the substrate with the previouslydescribed solution (e.g., a solution comprising at least one acid, atleast one ingredient comprising a fluorine ion, ethylene glycol, andwater), wherein the substrate is contacted with the solution for a timesufficient to remove metal from the substrate with substantially nosubstrate erosion.

For example, there is disclosed a method for selectively removing metalfrom a substrate comprising aluminum and being at least partiallycovered with a metal. In this example, the method may comprisecontacting the substrate with a solution comprising, in % weight of thesolution, 1-20%, such as 10-15% HF, 1-40%, such as 10-15% NH₄F, 40-95%,such as 50-60% ethylene glycol, and the balance of water.

According to some exemplary processes, the substrate may be contactedwith the solution for a time sufficient to remove metal from thesubstrate without substantially reacting with the substrate, wherein analuminum ethoxide layer is formed on at least one portion of exposedsubstrate.

The contact time sufficient to remove metal from the substrate may bedependent on various factors, including the thickness of the metal layerto be removed, the concentration and amount of the ingredients present,such as the acid, the ingredient comprising a fluorine ion, ethyleneglycol and the water. Other factors that may affect the contact timeinclude the pH of the solution, and temperature at which the solution iscontacted to the substrate. Generally, the etching rate may decreasewith increased pH. Thus, the contact time may increase with increase pHof the solution.

The etch rate may also be a function of the solution temperature, theetch rate generally increasing with increasing temperature. The methoddescribed herein may be performed with the solution heated up to 120° F,wherein the maximum temperature allowable may depend on the selectivityof the given system. For example, in one embodiment, in which Ta/TaN isremoved from an Al containing substrate, the solution is heated to atemperature ranging from 85° C. to 95° C. In another embodiment, thesolution is heated to a temperature ranging from room temperature up to85° C.

The above-described method may be carried out using any well-known wetprocessing procedure. For example, contacting the metal coated substratewith the described solution may take place using a batch-type procedure,for example, by immersing the substrate in a bath containing thesolution for a period of time sufficient to remove the metal.

Alternatively, contacting the substrate may be carried out using acontinuous procedure. In this embodiment, one or more substrates may bepassed through a vessel containing the solution at a rate that willremove metal.

Either one or both of these types of processes may occur in a single ormulti-step processes. In addition, a process comprising a mixture of acontinuous and batch style may be used.

Regardless of the exact process used, the mechanism may be generally thesame. For example, the processing solution may contain a complexingagent in quantities sufficient to react with the substrate surface assoon as the coating is removed, i.e., to expose the substrate. Thisorganic—metallic complex may be non-soluble in the stripping medium,which allows it to protect the substrate from attack, but may besubsequently removed during further processing. For example, thealuminum ethoxide complex may be soluble in water and may be removedwith a rinse following the acidic removal of the coating.

In one embodiment, the process comprises exposing the substrate to atimed immersion in a wet chemical bath or a spray from a chemical spraytool until the metal becomes completely removed. Such a process can bemulti-step in so far as there may be a cycle ofstrip-inspect-strip-inspect until removal is complete. While acontinuous system is generally rare in the semiconductor industry, itcould be operated as described herein once the strip time is establishedfor a particular set of components.

Other than in the operating examples, or where otherwise indicated, allnumbers expressing quantities of ingredients, reaction conditions, andso forth used in the specification and claims are to be understood asbeing modified in all instances by the term “about.” Accordingly, unlessindicated to the contrary, the numerical parameters set forth in theforegoing specification and following claims are approximations that mayvary depending upon the desired properties sought to be obtained. At thevery least, and not as an attempt to limit the application of thedoctrine of equivalents to the scope of the claims, each numericalparameter should be construed in light of the number of significantdigits and ordinary rounding approaches.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the subject matter describedherein. Thus, it should be understood that the invention is not limitedto the discussion set forth in the specification. Rather, the presentinvention is intended to cover modifications and variations.

1. A solution comprising, in weight % of the solution: 1-20% HF, buffered HF, or mixtures thereof, 1-10% of an ingredient selected from the group consisting of ammonium fluoride (NH₄F), potassium fluoride, sodium fluoride and mixtures thereof, approximately 50-60% ethylene glycol, and water.
 2. The solution of claim 1, comprising 10-15% of the HF, buffered HF, or mixtures thereof.
 3. The solution of claim 1, wherein the ingredient is present in amount ranging from 1-5%, by weight, of the solution.
 4. The solution of claim 1, wherein said solution has a pH of less than
 6. 5. The solution of claim 1, further comprising at least one anionic, nonionic, cationic or amphoteric surfactant.
 6. The solution of claim 5, wherein the at least one surfactant is chosen from primary, secondary and tertiary amines of chain length C₂ to C₁₂, amides, fluorocarbons and alkylethoxylates.
 7. A solution comprising, in weight % of the solution: 1-20% HF, buffered HF, or mixtures thereof, 1-5% NH₄F, approximately 50-60% ethylene glycol, a surfactant, and the balance of water. 